Frangible well plugs are commonly used in tools for oil and/or gas wells. These plugs provide a pressure barrier in the tool, for example during periodic or permanent isolation of zones in the well, during well integrity testing, etc.
These frangible well plugs have a frangible barrier element in the form of a frangible disc made from glass, hardened glass, ceramics etc. The barrier element is provided in a seat in a metal housing. The barrier element may be removed by means of various techniques, where the purpose is to disintegrate the element into small pieces.
An example of a glass plug is known from NO 321 976 (TCO AS). The plug comprises a number of layered or stratified ring discs of a given thickness, which are placed in abutment on top of one another. Between the different layers of the plug an intermediate film of plastic, felt or paper is inserted; the various glass layers may also be joined by means of lamination by an adhesive such as a glue. During use the plug will be mounted in a plug-receiving chamber in a tubing, where the underside of the plug rests in a seat at the bottom of the chamber. An explosive charge is furthermore incorporated in the top of the plug by one or more recesses being drilled out from the top of the plug, in which recesses the explosive charge(s) are placed.
Another example is known from NO 20130427 (Vosstech AS). Here, the plug has one glass disc, which may be disintegrated by a radial pin or loading device being pushed into the glass disc.
One object of the invention is to provide a well plug with one glass disc body at higher pressure ratings.
With the above prior art well plugs, different types of seals are used between the metal and the glass. Often, one type of seal (typically o-ring) is used circumferentially around the glass disc to avoid fluid flow in the area between the glass disc and the metal housing. A second type of seal is used in the upper part and lower part of the seat to avoid contact between the glass disc and the metal housing, as is it known for the skilled person that such contact will cause an undesired breaking of the glass disc when the differential fluid pressure is increasing above a certain level. This second type of seal is often referred to as a force transmitting device, for transmitting the force applied to the glass by the fluid pressure further to the housing.
At high differential pressures above the glass disc, a relatively high force is applied by this fluid pressure onto the glass disc, where this relatively high force is transmitted to the seat of the housing. At such high pressures, the glass disc itself may be deformed slightly, increasing the risk for contact between the glass and the metal. Another object of the invention is to provide a force transmitting device which increases the possible differential pressure over the glass disc.
Today, a very high precision is necessary for the chamfered surfaces of the glass disc and the chamfered surfaces of the seat. The hardening process of the glass disc comprises heating and subsequent cooling of the glass disc, which may cause the glass disc to become slightly uneven. For some prior art application, it has been found necessary to polish the surfaces of the glass disc, a process which may damage the glass disc. Hence, one object of the invention is to reduce the need for precision of the chamfered surfaces of the glass disc and the chamfered surfaces of the seat.